The mouse Brachyury gene (T) is required in notochord differentiation and posterior mesoderm formation during axial development. We have isolated the chick homologue of T(Ch-T) and determined its putative protein sequence and expression pattern during embryogenesis. Ch-T is expressed in the epiblast close to and within the primitive streak, in early migrating mesoderm and in the notochord. In later stages Ch-T expression is found in the tail bud and in the entire notochord. The notochord expression ceases in an anterior-posterior wave when the formation of the body anlage is completed. This pattern is consistent with those reported for the expression of the mouse T gene and the T homologues of Xenopus laevis and zebrafish, suggesting that the mechanisms of embryonic pattern formation are highly conserved in all vertebrates. The N-terminal half of Ch-T shows a very high degree of sequence identity with the corresponding region of mouse T which has DNA-binding activity, and with the N-terminal half of Xenopus (Xbra) and zebrafish (Ntl) T protein. Finally, we have analyzed the effects of activin A on Ch-T induction and axis formation. Localized activin A treatment of prestreak blastoderms results in ectopic Ch-T expression that correlates with formation of second primitive streaks or with repositioning of the site of single streak origin (Cooke et al., 1994). These results strengthen the previous evidence that Brachyury activation is an early response to axis-inducing signals in vivo.